Pairs or triplets of associated charged particles, separated by distances of a few hundred nanometers, have been found to be more effective at both cell killing and at producing chromosome aberrations than randomly incident particles. Using particles accelerated by the Radiological Research Accelerator Facility at Brookhaven National Lab., it has been found that the biological effectiveness of paired associated particles decreases as the particle separation increases. The dependence on particle separation is highly skewed, with most of the interactions occurring over distances of a fraction of a micron, but there is a small component acting over much larger distances in excess of a micron. The principal thrust of experiments in the coming year will be to investigate the effect of increasing further the spacing between paired ions. The maximum particle separation attainable depends on the maximum accelerating energy of the facility and the limit has already been reached at Brookhaven. Consequently it will be necessary to move the experiment to Los Alamos Scientific Laboratory where a higher energy van de Graaff accelerator is available capable of accelerating molecular beams. The long term objective of these investigations is to elucidate the influence of the spatial distribution of energy absorption on biological effects that include cell lethality, the production of chromosome aberrations and oncogenic transformation. This information, in turn, will shed light on the basic mechanisms involved.